The problem with generic seat belt retractor tensioning devices of the pyrotechnically activated-type is that the pressure conditions during the tensioning operation vary considerably. In particular, very high pressure peaks can damage parts of the tensioning device, or disturb the motion sequences of the tensioning device. Furthermore, the problem is that, unless the safety belt is provided with a force limiting device, the belt force at the start of the force-limited belt extension motion following the tensioning operation increases for a short time to a force limiting level that is higher than that defined by the force limiting device due to the pressure still prevailing in the tensioning device. This effect is generally designated as a force limiting disturbance. In order to prevent these disadvantages resulting from excessive pressure, different solutions have been devised to prevent pressure peaks and reduce the pressure following the tensioning of the safety belt as quickly as possible.
U.S. Pat. No. 6,345,504 describes a tensioning device in which the piston is formed by an elastic ball with a blind opening, and a through-opening is provided in a subsequent ball through which a rod held in the blind opening protrudes. In case of an above-normal pressure, the rod causes the elastic ball to break in order to allow the gas to escape through the gas channel. The production of both the blind opening and the through-opening is expensive and requires a complex assembly process which further increases the price of the tensioning device.